The RTH 100 Series of temperature and humidity circular chart recorders uses the proven design and combination of bimetallic coils and hygroscopic membrane for the direct measurement of temperature and humidity.

Changes in atmospheric temperature and humidity are detected by the two elements within the recorder. The bimetallic coil monitoring the temperature and the hygroscopic element the humidity.

Air is drawn through the instrument and over the two sensing elements by an electrically driven fan. Expansion, or contraction, of the bimetallic coil, due to temperature change, causes pen movement by direct linkage. Similarly a change in humidity causes extension, or contraction, of the hygroscopic membrane and direct linkage again causes movement of the second pen.

Fitted to the pen arms are disposable fiber-tipped pens which are readily replaceable without mess. Each of the two pens traces in a different color for ease of recognition.

The RTH 100 is available in single pen record only format to monitor either temperature or humidity only, or in a two pen version to monitor both variables. Alternatively each of the above options can be supplied as a recorder/controller fitted with up to four independent alarm contacts, each with its own setting pointer adjustable over the full chart range. Each alarm contact operates an output relay providing double pole changeover, voltage free, output contacts. oth high and low alarm points for temperature and humidity are indicated by illumination of red LED'S, suitably labeled, situated in the recorder door.

A further option on recorder/controller instrument is the availability of an audible alarm indication of set points, silenced by operation of a mute button located on the door adjacent to the alarm LED'S.

Each instrument can be supplied for either of two modes of sampling: the most commonly used ambient sampling mode, monitoring the atmosphere adjacent to the recorder; or remote sampling mode, where a continuous sample is extracted (e.g. through the wall of a room, or environmental chamber) and passed through the instrument. Options are available to return the sample to the environment from which it was extracted, or to atmosphere surrounding the recorder.